Voltage controlled highly linear resistive elements

ABSTRACT

Controllable resistance elements and methods of setting the same include a junction field effect transistor configured to provide a resistance on a signal line. A first pass transistor is configured to apply a charge increment or decrement to the junction field effect transistor responsive to a control pulse, such that the resistance on the signal line changes.

BACKGROUND Technical Field

The present invention generally relates to neuromorphic circuits and,more particularly, to voltage-controlled resistive elements inneuromorphic circuits.

Description of the Related Art

An artificial neural network (ANN) is an information processing systemthat is inspired by biological nervous systems, such as the brain. Thekey element of ANNs is the structure of the information processingsystem, which includes a large number of highly interconnectedprocessing elements (called “neurons”) working in parallel to solvespecific problems. ANNs are furthermore trained in-use, with learningthat involves adjustments to weights that exist between the neurons. AnANN is configured for a specific application, such as patternrecognition or data classification, through such a learning process.

Referring now to FIG. 1, a generalized diagram of a neural network isshown. ANNs demonstrate an ability to derive meaning from complicated orimprecise data and can be used to extract patterns and detect trendsthat are too complex to be detected by humans or other computer-basedsystems. The structure of a neural network is known generally to haveinput neurons 102 that provide information to one or more “hidden”neurons 104. Connections 108 between the input neurons 102 and hiddenneurons 104 are weighted and these weighted inputs are then processed bythe hidden neurons 104 according to some function in the hidden neurons104, with weighted connections 108 between the layers. There may be anynumber of layers of hidden neurons 104, and as well as neurons thatperform different functions. There exist different neural networkstructures as well, such as convolutional neural network, maxoutnetwork, etc. Finally, a set of output neurons 106 accepts and processesweighted input from the last set of hidden neurons 104.

This represents a “feed-forward” computation, where informationpropagates from input neurons 102 to the output neurons 106. Uponcompletion of a feed-forward computation, the output is compared to adesired output available from training data. The error relative to thetraining data is then processed in “feed-back” computation, where thehidden neurons 104 and input neurons 102 receive information regardingthe error propagating backward from the output neurons 106. Once thebackward error propagation has been completed, weight updates areperformed, with the weighted connections 108 being updated to accountfor the received error. This represents just one variety of ANN.

One challenge in implementing ANNs lies in the structure of the weights.When forming weight structures in dedicated hardware systems, existingweights lack linearity and/or are formed with exotic materials.

SUMMARY

A controllable resistance element includes junction field effecttransistor configured to provide a resistance on a signal line. A firstpass transistor is configured to apply a charge increment or decrementto the junction field effect transistor responsive to a control pulse,such that the resistance on the signal line changes.

A weight array includes a plurality of controllable resistance elements.Each controllable resistance element includes a junction field effecttransistor configured to provide a resistance on a signal line and afirst pass transistor configured to apply a charge increment ordecrement to the junction field effect transistor responsive to acontrol pulse, such that the resistance on the signal line changes. Aplurality of AND gate are configured to control addressing of theplurality of controllable resistance elements. The control pulse is onlypassed to a controllable resistance element when a respective AND gateis triggered.

A method of training a neural network includes applying an input signalto an array of weights to generate weighted output signals based onresistances of respective weights in the array of weights. A differencebetween the weighted output signals and a predetermined expected outputis determined. Weights in the array of weights are set by applying apulse to a controllable resistance element in each weight. The pulseincrements or decrements a charge on a junction field effect transistorin the respective controllable resistance element.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description will provide details of preferred embodimentswith reference to the following figures wherein:

FIG. 1 is a diagram of a general neural network;

FIG. 2 is a diagram of a neural network that employs controllableresistive elements as weights in accordance with the presentembodiments;

FIG. 3 is a block diagram of a neural network control circuit inaccordance with the present embodiments;

FIG. 4 is a circuit diagram of a controllable resistive element inaccordance with the present embodiments;

FIG. 5 is a circuit diagram of a controllable resistive element inaccordance with the present embodiments;

FIG. 6 is a circuit diagram of a controllable resistive element inaccordance with the present embodiments;

FIG. 7 is a block diagram of a weight array employing controllableresistive elements in accordance with the present embodiments;

FIG. 8 is a block diagram of a weight array employing controllableresistive elements in accordance with the present embodiments;

FIG. 9 is a block/flow diagram of a method of training a neural networkin accordance with the present embodiments; and

FIG. 10 is a block diagram of a neural network processing system inaccordance with the present embodiments.

DETAILED DESCRIPTION

Embodiments of the present embodiments implement artificial neuralnetworks (ANNs) using weight arrays that employ voltage-controlledmemristive elements. In specific embodiments, junction gate field effecttransistors (JFETs) are used to control the resistivity of the weightsin a predictable, linear fashion.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 2, an artificial neuralnetwork (ANN) architecture 200 is shown. It should be understood thatthe present architecture is purely exemplary and that otherarchitectures or types of neural network may be used instead. Duringfeed-forward operation, a set of input neurons 202 each provide an inputvoltage in parallel to a respective row of weights 204. The weights 204each have a settable resistance value, such that a current output flowsfrom the weight 204 to a respective hidden neuron 206 to represent theweighted input. The current output by a given weight is determined as

${I = \frac{V}{r}},$

where V is the input voltage from the input neuron 202 and r is the setresistance of the weight 204. The current from each weight addscolumn-wise and flows to a hidden neuron 206. A set of reference weights207 have a fixed resistance and combine their outputs into a referencecurrent that is provided to each of the hidden neurons 206. Becauseconductance values can only be positive numbers, some referenceconductance is needed to encode both positive and negative values in thematrix. The currents produced by the weights 204 are continuously valuedand positive, and therefore the reference weights 207 are used toprovide a reference current, above which currents are considered to havepositive values and below which currents are considered to have negativevalues.

As an alternative to using the reference weights 207, another embodimentmay use separate arrays of weights 204 to capture negative values. Eachapproach has advantages and disadvantages. Using the reference weights207 is more efficient in chip area, but reference values need to bematched closely to one another. In contrast, the use of a separate arrayfor negative values does not involve close matching as each value has apair of weights to compare against. However, the negative weight matrixapproach uses roughly twice the chip area as compared to the singlereference weight column. In addition, the reference weight columngenerates a current that needs to be copied to each neuron forcomparison, whereas a negative matrix array provides a reference valuedirectly for each neuron. In the negative array embodiment, the weights204 of both positive and negative arrays are updated, but this alsoincreases signal-to-noise ratio as each weight value is a difference oftwo conductance values. The two embodiments provide identicalfunctionality in encoding a negative value and those having ordinaryskill in the art will be able to choose a suitable embodiment for theapplication at hand.

The hidden neurons 206 use the currents from the array of weights 204and the reference weights 207 to perform some calculation. The hiddenneurons 206 then output a voltage of their own to another array ofweights 207. This array performs in the same way, with a column ofweights 204 receiving a voltage from their respective hidden neuron 206to produce a weighted current output that adds row-wise and is providedto the output neuron 208.

It should be understood that any number of these stages may beimplemented, by interposing additional layers of arrays and hiddenneurons 206. It should also be noted that some neurons may be constantneurons 209, which provide a constant voltage to the array. The constantneurons 209 can be present among the input neurons 202 and/or hiddenneurons 206 and are only used during feed-forward operation. During backpropagation, the output neurons 208 provide a voltage back across thearray of weights 204. The output layer compares the generated networkresponse to training data and computes an error. The error is applied tothe array as a voltage pulse, where the height and/or duration of thepulse is modulated proportional to the error value. In this example, arow of weights 204 receives a voltage from a respective output neuron208 in parallel and converts that voltage into a current which addscolumn-wise to provide an input to hidden neurons 206. The hiddenneurons 206 provide combine the weighted feedback signal with aderivative of its feed-forward calculation and stores an error valuebefore outputting a feedback signal voltage to its respective column ofweights 204. This back propagation travels through the entire network200 until all hidden neurons 206 and the input neurons 202 have storedan error value.

During weight updates, the input neurons 202 and hidden neurons 206apply a first weight update voltage forward and the output neurons 208and hidden neurons 206 apply a second weight update voltage backwardthrough the network 200. The combinations of these voltages create astate change within each weight 204, causing the weight 204 to take on anew resistance value. In this manner the weights 204 can be trained toadapt the neural network 200 to errors in its processing. It should benoted that the three modes of operation, feed forward, back propagation,and weight update, do not overlap with one another.

Referring now to FIG. 3, a block diagram of a neuron 300 is shown. Thisneuron may represent any of the input neurons 202, the hidden neurons206, or the output neurons 208. It should be noted that FIG. 3 showscomponents to address all three phases of operation: feed forward, backpropagation, and weight update. However, because the different phases donot overlap, there will necessarily be some form of control mechanismwithin in the neuron 300 to control which components are active. Itshould therefore be understood that there may be switches and otherstructures that are not shown in the neuron 300 to handle switchingbetween modes.

In feed forward mode, a difference block 302 determines the value of theinput from the array by comparing it to the reference input. This setsboth a magnitude and a sign (e.g., + or −) of the input to the neuron300 from the array. Block 304 performs a computation based on the input,the output of which is stored in storage 305. It is specificallycontemplated that block 304 computes a non-linear function and may beimplemented as analog or digital circuitry or may be performed insoftware. The value determined by the function block 304 is converted toa voltage at feed forward generator 306, which applies the voltage tothe next array. The signal propagates this way by passing throughmultiple layers of arrays and neurons until it reaches the final outputlayer of neurons. The input is also applied to a derivative of thenon-linear function in block 308, the output of which is stored inmemory 309.

During back propagation mode, an error signal is generated. The errorsignal may be generated at an output neuron 208 or may be computed by aseparate unit that accepts inputs from the output neurons 208 andcompares the output to a correct output based on the training data.Otherwise, if the neuron 300 is a hidden neuron 206, it receives backpropagating information from the array of weights 204 and compares thereceived information with the reference signal at difference block 310to provide a continuously valued, signed error signal. This error signalis multiplied by the derivative of the non-linear function from theprevious feed forward step stored in memory 309 using a multiplier 312,with the result being stored in the storage 313. The value determined bythe multiplier 312 is converted to a backwards propagating voltage pulseproportional to the computed error at back propagation generator 314,which applies the voltage to the previous array. The error signalpropagates in this way by passing through multiple layers of arrays andneurons until it reaches the input layer of neurons 202.

During weight update mode, after both forward and backward passes arecompleted, each weight 204 is updated proportional to the product of thesignal passed through the weight during the forward and backward passes.The update signal generators 316 provide voltage pulses in bothdirections (though note that, for input and output neurons, only onedirection will be available). The shapes and amplitudes of the pulsesfrom update generators 316 are configured to change a state of theweights 204, such that the resistance of the weights 204 is updated.

In one particular embodiment, the weights 204 may be implemented inhardware, using relatively complicated weighting circuitry or usingresistive cross point devices. It is specifically contemplated that theweights 204 have a highly linear response. The weights 204 may belong toa class of device called a resistive processing unit (RPU), becausetheir non-linear characteristics are used to perform calculations in theneural network 200. The RPU devices may be implemented with resistiverandom access memory (RRAM), phase change memory (PCM), programmablemetallization cell (PMC) memory, or any other device that has non-linearresistive switching characteristics. Such RPU devices may also beconsidered as memristive systems. The present embodiments providedifferent RPU embodiments based on JFETs.

One important aspect of ANNs is the linearity of the resistive paththrough the weights 204. In the present embodiments, the resistance iscontrolled using a voltage or current source. Such sources can beimplemented as steady-state biases or as pulses. Symmetric updatecharacteristics are preferred for updates to the weights 204 where,after a positive and a subsequent negative pulse, the resistive elementshould return to the weight it had prior to the pulses. Linear variationof resistance or conductance with the number of pulses is also providedherein. Most existing approaches to achieving these effects includeexotic materials or have difficult-to-control resistive behaviors.Another drawback to existing voltage-controlled resistors is that thereis generally a threshold voltage needed to change the resistance.Computations using such devices need to keep their voltages below thisthreshold to avoid changing the resistance state, limiting theoperational range of the array.

The present embodiments use JFETs, with or without voltage feedback, toprovide voltage-controlled resistive (VCR) elements. Such elements maybe used in, e.g., neuromorphic arrays as the weights 204 or may be usedin any other appropriate application. Some embodiments include acapacitive element or other charge-integrating element in conjunctionwith the VCR to provide stepwise pulsed current control of the resistiveelement.

The VCR and optional capacitive element may be discrete components ormay be monolithically integrated into a single substrate. Trenchcapacitor structures may be used as the capacitive element, but thecapacitive element may alternatively include semiconductor devices suchas diodes and transistors or passive elements such as resistors andcapacitors to act as a sample-and-hold circuit for pulse mode controlover the resistive element. As a result, the present embodiments may beemployed to provide arrays of linearly resistive elements using at anyappropriate scale and with readily simulated circuit behavior. Thus thepresent embodiments provide no-threshold resistive control, largeoperational range (e.g., between about −5 V and +5 V or greater), andvoltage-independent resistance for the signal voltages.

Referring now to FIG. 4, a voltage-controlled resistance circuit 400 isshown. The circuit 400 includes inputs for a V+ voltage source 402, a V−voltage source 404, and a pulse control 406. Pass transistors 408 aretriggered by the pulse control 406 to pass a fixed, predictable amountof charge into the circuit. A capacitor 410 integrates the currentpassing through resistor 416 and stores the charge, which provides thegate bias of a JFET 412. A sense resistor 414 acts as a voltage divider.In an alternative embodiment, the sense resistor 414 may be omitted,with the collector of the JFET 412 being left open.

The pulse applied by pulse control 408 may be positive or negative,activating a respective nFET or pFET from the pass transistors 408,which may be implemented in some embodiments as metal oxidesemiconductor field effect transistors (MOSFETs). In alternativeembodiments, the pass transistors 408 may be implemented as JFETs. Inthis embodiment, the pass transistors 408 are arranged back-to-back in acomplimentary configuration.

When a positive pulse arrives at pulse control 408, the nFET exposes thepositive voltage 402 to add charge to the circuit. When a negative pulsearrives at pulse control 408, the pFET exposes the negative voltage 404to remove charge from the circuit. In this manner, an amount of chargeis held at the gate of the JFET 412, causing the JFET 412 to act as avariable resistor having a resistance that is linearly determined by theamount of charge.

The gate region of the JEFT 412 forms a semiconductor p/n junction (or ametal/semiconductor junction in the case of a metal-semiconductor fieldeffect transistor (MESFET)). Because there is only one charge carrierpresent in the JFET 412, when the gate is reverse-biased, the carrierconcentration under the gate is reduced as the depletion layer grows.This behavior is distinct from that of bipolar transistors and MOSFETs.JFETs also have a high input impedance (e.g., in excess of about 10¹²ohms). Due to this high input impedance, if a low-leakage capacitor 410is coupled to the gate, any charge injected into the capacitor will havea large retention time and will therefore be quasi-permanent. Ingeneral, a p-channel JFET needs a positive voltage at the gate toincrease source/drain resistance, while an n-channel JFET needs anegative charge at the gate to increase source/drain resistance.

The JFET 412 should have a large resistance range. It is specificallycontemplated that the JFET 412 should have a relatively large basesource/drain resistance. As neural networks grow in size, largerresistances are useful to keep the current draw low. Thus, it iscontemplated that the JFET 412 may have a base source/drain resistancegreater than about 50 kΩ and, in some cases, this resistance may bemeasured in megaohms. The JFET 412 will have a resistance range of,e.g., about 10× variation to about 50× variation.

Referring now to FIG. 5, a voltage-controlled resistance circuit 500 isshown. In this embodiment, instead of connecting the drains of the passresistors 504 to respective+ and—power supplies, the drains areconnected to the common gate node. This simplifies the wiring by usingthe pulse signal 502 itself as the current/charge source. In practice,the current control resistor 508 may have a resistance of, e.g.,multiple megaohms. This configuration reduces leakage current, becausethe backward leakage path would be along the pulse bus line rather thanthe power supplies. Leakage current can be further reduced if the pulsesource 502 is in an “open collector” configuration and floats betweenpulses. The pulse source 502 should have a stiff enough voltage sourcethat it does not suffer voltage droop in larger arrays where the currentdraw can be large.

Referring now to FIG. 6, a voltage-controlled resistance circuit 600 isshown. Notably only a single pass transistor 602 is employed, gated byan “enable” signal 606 and with a terminal connected to a pulse bus 608.As before, the output of the pass transistor 602 adds or removes acharge at the gate of the JFET 604. This circuit 600 uses only a singlepolarity (e.g., positive), which eliminates the need for a second powersupply rail and wiring. Although a p-channel JFET 604 is shown in thisspecific embodiment, alternative embodiments may replace the p-channelJFET 604 and its positive power supply with an n-channel JFET that ischarged using a negative power supply. Rather than charging thecapacitor 610 with a pulse having a first polarity and discharging withpulse having a second polarity, discharge is performed by grounding thepulse bus 608.

Referring now to FIG. 7, an array 700 of weights 204 is shown. Eachweight 204 includes a VCR 702, an AND gate 704, and an access transistor706 that may be implemented as a FET. In this embodiment, the VCRs 702may be implemented as set forth in FIGS. 4 and 5 above. Each weight 204receives an input from a pulse bus 707, a first pulse address 710, and asecond pulse address 708. The pulse addresses 708 and 710 feed into theAND gate 704 to trigger the passage of a pulse from pulse bus 707 intothe VCR 702, which adjusts its resistance in the manner described above.In this manner, specific weights 204 can be addressed for respectivepositive and negative pulses to adjust the weighting applied to inputsignals.

During operation, the VCR 702 has an input path 714 and an output path712 that carries input signals from input neurons 202 and passesweighted signals onward toward the output neurons 208. The weightedsignal has a voltage drop relative to the input signal based on thepresent resistance of the VCR 702.

Referring now to FIG. 8, an array 800 of weights 204 is shown. In thisembodiment, the weights 204 are implemented with a VCR 802 such as thatdescribed with respect to FIG. 6 above. The weights 204 include an ANDgate 804, but omit an access transistor. The VCR 802 accepts an inputfor pulse bus 807 as well as address signals 808 and 810. When bothaddress signals 808 and 810 are on at the same time, the AND gate 804provides an enable signal to the VCR 802 that triggers the introductionof new charge from the pulse bus 807. Alternatively, if the pulse bus807 is grounded, charge will be removed from the VCR 802. As with theabove embodiments, during operation input signals on input path 814 areweighted by the VCR and the weighted signals are output on output path812.

Referring now to FIG. 9, a method for training an array 700 or 800 in aneural network is shown. Block 902 sets the weights 204 by energizingthe pulse address lines (e.g., 708/710 or 808/810) for a particular VCR702 or 802. Block 904 emits a pulse on pulse bus 707/807 to change theresistance of the VCR 702 or 802, applying a fixed charge to the JFET inthe VCR 702/802. In the context of FIGS. 4 and 5, this pulse may bepositive or negative. In the context of FIG. 6, the pulse is only onepolarity, with the removal of charge being performed by grounding thepulse bus 807. Block 908 selects the next VCR 702/802 in the array700/800 and the process continues until all weights are set.

Block 910 then provides an input signal to the array 700/800, with theinput signal being weighted by the resistance of each weight 204, suchthat the output signals have amplitudes determined by the weights 204.After this calculation is performed, block 912 determines how much themagnitude of the weights 204 should be changed. It is specificallycontemplated that a control module may determine the error signals andmay furthermore control the setting of the weights 204 in block 902.Alternatively, a distributed, localized alteration of the weightsthrough backpropagation may be employed. Processing returns to block 902to set the weights accordingly. After the neural network is trained,calculations can be performed by applying an input signal and measuringthe output.

The present invention may be a system, a method, and/or a computerprogram product at any possible technical detail level of integration.The computer program product may include a computer readable storagemedium (or media) having computer readable program instructions thereonfor causing a processor to carry out aspects of the present invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present invention, as well as other variations thereof, means that aparticular feature, structure, characteristic, and so forth described inconnection with the embodiment is included in at least one embodiment ofthe present invention. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to FIG. 10, a neural network processing system 1000 isshown. The neural network processing system 1000 includes a hardwareprocessor 1002 and memory 1004. In addition, a neural network 1006 isimplemented using, e.g., the arrays of weights 204 described above. Theneural network 1006 may be implemented entirely in hardware or may be acombination of hardware elements and software elements. The neuralnetwork processing system 1000 also includes one or more functionalmodules. The functional modules may be implemented as software that isstored in memory 1004 and executed by processor 1002. In alternativeembodiments, the functional modules may be implemented as one or morediscrete hardware components in the form of, e.g., application specificintegrated chips or field programmable gate arrays.

An error module 1008 is used during training of the neural network 1006to determine how much a measured output differs from the expectedoutput. A training module 1010 then addresses the weights 204 in theweight arrays 700/800 and applies an appropriate pulse to change theresistance of each respective weight 204. This is continued untiltraining is complete, at which point the neural network 1006 can be useddirectly to perform computations.

Having described preferred embodiments of a system and method (which areintended to be illustrative and not limiting), it is noted thatmodifications and variations can be made by persons skilled in the artin light of the above teachings. It is therefore to be understood thatchanges may be made in the particular embodiments disclosed which arewithin the scope of the invention as outlined by the appended claims.Having thus described aspects of the invention, with the details andparticularity required by the patent laws, what is claimed and desiredprotected by Letters Patent is set forth in the appended claims.

1. A controllable resistance element, comprising: a junction fieldeffect transistor configured to provide a resistance on a signal line;and a first pass transistor configured to apply a charge increment ordecrement to the junction field effect transistor responsive to acontrol pulse, such that the resistance on the signal line changes. 2.The controllable resistance element of claim 1, further comprising asecond pass transistor connected to the first pass transistor in acomplimentary configuration.
 3. The controllable resistance element ofclaim 1, further comprising a first voltage source at a first voltagepolarity and a second voltage source at a second polarity, wherein thecontrol pulse triggers the pass transistor to expose the first or secondvoltage source to the junction field effect transistor depending on apolarity of the control pulse.
 4. The controllable resistance element ofclaim 1, further comprising a capacitor connected in parallel with agate of the junction field effect transistor.
 5. The controllableresistance element of claim 1, further comprising a resistor connectedin series between the pass transistor and the junction field effecttransistor.
 6. The controllable resistance element of claim 5, whereinthe resistor has a resistance over one megaohm.
 7. The controllableresistance element of claim 1, wherein the junction field effecttransistor is a p-channel junction field effect transistor.
 8. Thecontrollable resistance element of claim 1, wherein the junction fieldeffect transistor is an n-channel junction field effect transistor.
 9. Aweight array, comprising: a plurality of controllable resistanceelements, each controllable resistance element comprising: a junctionfield effect transistor configured to provide a resistance on a signalline; and a first pass transistor configured to apply a charge incrementor decrement to the junction field effect transistor responsive to acontrol pulse, such that the resistance on the signal line changes; anda plurality of AND gate configured to control addressing of theplurality of controllable resistance elements, wherein the control pulseis only passed to a controllable resistance element when a respectiveAND gate is triggered.
 10. The weight array of claim 9, wherein eachcontrollable resistance element further comprises a second passtransistor connected to the first pass transistor in a complimentaryconfiguration.
 11. The weight array of claim 9, further comprising anaccess transistor that is triggered by the AND gate and applies thecontrol pulse to an addressed controllable resistance element.
 12. Theweight array of claim 9, wherein each controllable resistance elementfurther comprises a first voltage source at a first voltage polarity anda second voltage source at a second polarity, wherein the control pulsetriggers the pass transistor to expose the first or second voltagesource to the junction field effect transistor depending on a polarityof the control pulse.
 13. The weight array of claim 9, wherein eachcontrollable resistance element further comprises a capacitor connectedin parallel with a gate of the junction field effect transistor.
 14. Theweight array of claim 9, wherein each controllable resistance elementfurther comprises a resistor connected in series between the passtransistor and the junction field effect transistor.
 15. The weightarray of claim 14, wherein the resistor has a resistance over onemegaohm. 16-20. (canceled)